It has long been recognized that vehicles having ground engaging/propulsion systems which utilize track have higher traction in soil and cause less ground compaction than vehicles of comparable weight equipped with propulsion systems utilizing solid or pneumatic-tire-equipped wheels. The "footprint" or engagement area of track propulsion systems with the ground is larger than the footprint which is practically achievable by wheel propulsion systems due to the footprint of each wheel being relatively small. In an attempt to compensate for such reduced footprint, large wheel vehicles such as agricultural tractors often have four and sometimes six wheels mounted on each axle. Propulsion systems for agricultural applications today, however, nearly exclusively utilize pneumatic tires rather than track. Present day efforts to increase traction and reduce soil compaction have centered around optimizing wheel propulsion systems even though track systems inherently provide such objectives. As stated in the Agricultural Engineer, Winter 1982 Edition, page 109, "Now that the pneumatic tyre provides the almost universal means of obtaining traction in agriculture, it is timely to incorporate a system of in-work tyre pressure control which will enable tyre deflection levels to be kept constant throughout the working cycle. The quest for increased traction with reduced levels of soil compaction makes the adoption of such a system increasingly desirable."
The popularity and nearly universal acceptance of wheel propulsion systems rather than track systems in agricultural use stems primarily from the present day track system's relatively higher noise levels, higher initial cost, lower maximum travel speed, and inability to transport itself on improved road surfaces without inflicting unacceptable damage. While the noise level and roading problems exhibited by conventional track may be minimized by cushioning the ground engaging surface thereof, commercially unacceptably high wear rates generally occur at the moveable joints between the rigid track sections when such conventional track is used in high speed applications.
Those skilled in the art have recognized that the foregoing advantages of wheel and track propulsion systems could be realized by utilizing a propulsion system in which a continuous rubber belt is entrained about a pair of wheels. Problems encountered in actually reducing such belt system to practice include how to drive such belt with the entrained wheels, how to maintain structural integrity of the belt and wheels, how to retain the belt in lateral alignment with the wheels when the wheels are subjected to large lateral loads, how to provide long life for the belt and wheels, and how to accommodate debris ingestion between the wheels and belt while maintaining the driving relationship therebetween without damaging either.
Limited success has been achieved in providing belt systems for some light duty applications such as snowmobiles. Light duty belt systems utilizing positive drives are illustrated, by example, in U.S. Pat. Nos. 3,510,174 and 3,858,948 which issued May 5, 1970 and Jan. 7, 1975, respectively. An example of a light duty vehicle utilizing friction drive is illustrated in U.S. Pat. No. 4,198,103 which issued Apr. 15, 1980. Attempts to expand the use of belt systems to heavy-duty commercial applications have, in general, met with failure. The following U.S. patents are directed toward resolving the previously described belt system problems by utilizing positive belt drives and applying them to heavy-duty applications: U.S. Pat. No. 2,338,817 which issued Jan. 11, 1944; U.S. Pat. No. 2,461,849 which issued Feb. 15, 1949; U.S. Pat. No. 3,703,321 which issued Nov. 21, 1972; and U.S. Pat. No. 4,241,956 which issued Dec. 30, 1980. The '849 patent which is assigned to B. F. Goodrich explained why positive drive was preferred over others: "It has also been proposed to eliminate the use of cross bars and to drive the track by friction alone, but use of such a construction requires a great area of angular contact or `wrap` about the driving wheel and confines the use of such tracks to drives for light vehicles providing great angular contact of the drive wheel. "
Notwithstanding the '849 recommendation, still other U.S. patents sought to apply friction drive to heavy-duty applications: U.S. Pat. No. 2,476,828 issued Jul. 19, 1949; U.S. Pat. No. 2,476,460 issued Jul. 19, 1949; U.S. Pat. No. 2,998,998 issued Sep. 5, 1961; U.S. Pat. No. 1,411,523 issued Apr. 4, 1922; U.S. Pat. No. 2,537,745 issued Jan. 9, 1951; U.S. Pat. No. 2,531, 111 issued Nov. 21, 1950; U.S. Pat. No. 2,494,065 issued Jan. 10, 1950; U.S. Pat. No. 2,429,242 issued Oct. 21, 1947; U.S. Pat. No. 2,350,076 issued May 30, 1944; U.S. Pat. No. 2,898,965 issued Aug. 11, 1959; U.S. Pat. No. 2,644,539 issued Jul. 7, 1953; U.S. Pat. No. 3,369,622 issued Feb. 20, 1968; and U.S. Pat. No. 4,281,882 issued Aug. 4, 1981. Other friction drive systems are shown in Otter Tractor Corporation advertising circular, U.K. Patent 1,604,615 published Dec. 9, 1981; U.K. Patent 2,048,800B published Jan. 12, 1983; U.K. Patent 278,779 published Oct. 20, 1927; Netherlands Patent 7,605,810 published Nov. 30, 1977; and German Patent 678,785 granted Jun. 29, 1939. Many of the aforementioned friction drive systems have a dual purpose driving/guiding structure which utilizes a driving slot having sloped lateral facing side surfaces and a belt having cooperatively sloped, laterally facing side surfaces which are frictionally engaged with the slot's side surfaces through an interference fit similar to V-belt drives. The driving slots of such structures tend to accumulate debris which disengages the frictionally engageable side surfaces. Radial grooves in the walls bounding such slots have been used in attempts to expel debris from the slots but have been generally ineffective.
The aforementioned patents are representative of a large body of patents which purport to solve one or more of the belt system implementation problems. Such body of patents constitutes documentary evidence that efforts to achieve this blend of track and wheel propulsion systems have been exerted for over half a century without realizing any practical measure of success. Solutions to the problems of actually implementing a heavy-duty vehicular belt drive system have proven ellusive and scientific scaling techniques have not, to date, been successfully applied to light duty vehicles for purposes of developing a heavy-duty belt system. Thus, despite the long felt need for and the advantages thereof, a heavy-duty application vehicle utilizing such belt system is commercially unavailable today.
It is, thus, the objective of this invention to provide a workable solution to the problems by taking into account that such vehicle's undercarriage, to be truly useful, should be roadable, provide high traction and low ground compression, and minimally disturb the underlying terrain, as well as operate in the heavy-duty working mode and provide a smooth ride for the operator in most soil conditions and topography from level land to steep inclinations while performing useful work without breaking the belts, losing drive capability between engaged wheels and belts, or disengaging the belts from the wheels.